RDMA/cxgb3: Don't exceed the max HW CQ depth

The max depth supported by T3 is 64K entries.  This fixes a bug
introduced in commit 9918b28d ("RDMA/cxgb3: Increase the max CQ
depth") that causes stalls and possibly crashes in large MPI clusters.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Cc: <stable@kernel.org>
Signed-off-by: Roland Dreier <rolandd@cisco.com>